1. Field of the Invention
The present invention relates to a communication system. More particularly, the present invention relates to an apparatus and method for transmitting/receiving signals in a communication system using a Hybrid Automatic Repeat reQuest (HARQ).
2. Description of the Related Art
As generally known in the art, a fundamentally important issue in the field of communication is to what extent data can be transmitted efficiently and reliably via channels. In line with the ever-increasing demand for faster communication systems capable of processing and transmitting various types of information (e.g. images, radio data), as well as providing conventional voice communication, extensive study is being made to improve system efficiency by adopting a channel coding scheme suitable for each system.
It is inevitable that, during data transmission, errors occur due to noise, interference, or fading depending on the channel condition and that such errors cause information loss. In order to reduce such information loss, it is customary to employ various error control schemes based on channel characteristics so that the system reliability improves.
Error control schemes for communication systems are generally classified into a Forward Error Correction (FEC) scheme and an Automatic Repeat reQuest (ARQ) scheme. The FEC scheme employs a code having error correction capability to correct the error of received information. The FEC scheme is used if there is no feedback channel for notifying the transmitting end whether information transmission has succeeded or failed. The ARC scheme employs, for example, a Cyclic Redundancy Check (CRC) code having excellent error detection capability so that, if an error is detected from received information, a retransmission request is sent to the transmitting end via a feedback channel.
The FEC scheme has a problem in that, if the receiving end fails to correct errors, erroneous information is delivered to users without correction. If a highly reliable system is to be implemented to avoid such a problem, a large number of codes must be used to correct errors. This increases the complexity of the decoding process and renders the implementation difficult.
Although the ARQ scheme has the advantage of simple structure and high reliability, it has a serious problem in that, as the channel error rate increases, the amount of processed information decreases abruptly. In order to overcome the problems of the ARQ and FEC schemes, they are properly combined to provide a HARQ scheme.
The HARQ scheme is classified into type I, type II, and type III systems.
The type I HARQ system has the simplest hybrid structure and encodes transmitted information words for error detection and error correction. A system classified as a type I HARQ system is further classified into a type of system in which a single code conducts both error detection and error correction, and another type of system in which two different codes respectively conduct error detection and error correction. The type I HARQ system has a problem in that, as the channel error rate increases, the amount of processed information decreases abruptly and that, if a single error correction code is used, overhead occurs since a predetermined amount of parity bits must be transmitted for error correction regardless of the channel condition.
The type II HARQ system does not discard packets, even if an error has been detected. Instead, additional parities are solely retransmitted and are combined with the packets. This increases the decoding efficiency and avoids the above-mentioned drawback.
The type III HARQ system also retransmits additional parities and combines them with erroneous packets so that the decoding efficiency improves, as in the case of the type II HARQ system. In general, packets to be retransmitted by the type II HARQ system include additional parities alone and, if there is a retransmission request, additional packets that have not been sent are transmitted in order, combined with previously received packets, and decoded.
The HARQ operation in the type II HARQ system will now be described in more detail.
Devices adapted for HARQ transmission/reception will hereinafter be referred to as HARQ entities. The transmitting-side HARQ entity is adapted to transmit and retransmit HARQ packets. The receiving-side HARQ entity is adapted to combine and soft-combine retransmitted HARQ packets and to transmit an Acknowledgement/Negative Acknowledgement (ACK/NACK) response. The transmitting/receiving HARQ entities consist of a number of HARQ processors, which are basic unit devices for transmitting/receiving user packets. The transmitting-side HARQ processors are adapted to transmit and retransmit user packets, and the receiving-side HARQ processors are adapted to receive, combine, and soft-combine user packets.
Pairs of HARQ processors exist in the transmitting and receiving sides, and each HARQ entity has a number of HARQ processors so as to enable continuous transmission/reception. Particularly, the HARQ processors transmit user packets, receive corresponding ACK/NACK information, and conduct retransmission. If only a single HARQ processor exists, it is not until user data is transmitted and corresponding ACK/NACK information is received that other packets can be transmitted. If a number of processors exist, other processors can transmit data while a processor waits to receive an ACK/NACK. This guarantees continuous transmission/reception.
The basic operation of the HARQ processors will now be described.
Transmitting-side HARQ processors channel-code user data, transmit it, and store it in a buffer in order to retransmit it at a later time. Upon receiving an ACK regarding the stored data, the data is flushed. Upon receiving a NACK regarding the data, the data is retransmitted according to a retransmission protocol. A retransmission protocol will be described later in more detail with reference to FIGS. 1A and 1B.
Receiving-side HARQ processors receive data via physical channels, channel-decode it and confirm if an error has occurred through a CRC. If an error has occurred, the data is stored in the buffer, and a NACK is transmitted. If retransmission data regarding the data is received later, the retransmitted data is combined or soft-combined with the data stored in the buffer in order, and is channel-decoded. The error check is then conducted again. If it is confirmed that the error still exists, a NACK is transmitted, and the above process is repeated. If the error has been cleared, an ACK is transmitted.
In summary, erroneous data is not discarded, but is combined or soft-combined with retransmitted data and is channel-decoded. In this manner, the HARQ operation reduces the BLock Error Rate (BLER) regarding packets.
A process for dividing a code word, which has been encoded by using a mother code, into packets of a predetermined length for retransmission in the type II HARQ system and a conventional retransmission protocol method will now be described with reference to FIGS. 1A and 1B.
FIG. 1A illustrates a code word for retransmission in the conventional type II HARQ system.
Referring to FIG. 1A, a code word composed of a mother code 100 is divided into packets of a predetermined length. It is assumed that the coding ratio of the mother code for channel coding is 1/4 and that the mother code has a systematic structure. The code word packet encoded by using the code word 100 has a length corresponding to 7200 symbols, and the length of the information word in the packet corresponds to 1800 symbols. The parity length corresponds to 5400 symbols. The code word packet is divided into three packets 111-113 each having a length of 2400 for retransmission in the type II HARQ system.
FIG. 1B shows a process for retransmission in the conventional type II HARQ system.
Referring to FIG. 1B, the transmitter sends the first packet 111 to the receiver in the initial transmission step 121. If the transmitter receives an ACK from the receiver in step 123, the process is terminated. If the transmitter received a NACK from the receiver in step 122 after transmitting the first packet 111 to the receiver in step 121, the transmitter sends the next packet, i.e. the second packet 112, in step 124.
If the transmitter receives an ACK from the receiver in step 126 after transmitting the second packet 112 to the receiver in step 124, the process is terminated. If the transmitter receives a NACK from the receiver in step 125 after transmitting the second packet 112 to the receiver in step 124, the transmitter transmits the next packet, i.e. the third packet 112, in step 127.
If the transmitter receives an ACK from the receiver in step 129 after transmitting the third packet 113 to the receiver in step 127, the process is terminated. If the transmitter receives a NACK from the receiver in step 128 after transmitting the third packet 113 to the receiver in step 127, the transmitter returns to step 121 and transmits the first packet 111, because the code word has no more packets to send. If the transmitter still receives a NACK, rather than an ACK, after the maximum number of allowable transmissions is reached, the transmission process is aborted. The maximum number of allowable transmissions is defined as the number of allowable retransmissions plus that of the initial transmission. If the transmission process is carried out on an assumption that the maximum number of allowable transmissions is three, the process is terminated after a NACK is received from the receiver in step 128.
The effective coding ratio, which is related to type II HARQ retransmissions according to the protocol shown in FIG. 1B, will now be described. The effective coding ratio is defined as the ratio between the length of an information word to be transmitted and the length of total packets actually transmitted. FIG. 1B will be referred to for more detailed descriptions. When the transmitter initially transmits the first packet 111, which includes an information word, in step 121, the length of the information word is 1800, the length of the packet is 2400, and the effective ratio is 3/4. When the transmitter sends the second packet 112 in step 124, the length of the information word is 1800, the length of total packets transmitted is 4800, and the effective coding ratio is 3/8. When the third packet 113 of the entire code word packet composed of a mother code is retransmitted, the length of the information word is 1800, the length of total packets transmitted is 7200, and the effective coding ratio is 1/4.
As such, the type II HARQ system uses the lowest coding ratio of 1/4 for the initial transmission and creates the entire code word packet. Then, the entire code word packet is properly divided and is transmitted at each retransmission request.
However, standardization committees (e.g. 3GPP LTE) stipulate that the BLER during initial transmission must be lower than 10%, which means that at least 90% of the initial transmission must proceed without error. Assuming that an LDPC code is used in such a case, the coding and decoding must be based on the lowest coding ratio of 1/4. This increases the complexity compared with the case of coding/decoding based on a high coding ratio. In addition, the LDPC code itself is not free in terms of the coding ratio. Therefore, the complexity can be substantially reduced if coding and decoding are conducted based on a corresponding coding ratio at the request of initial transmission and retransmission.